Cold mix paving compositions are made by mixing bitumen emulsion with aggregate at ambient temperature (i.e., temperature less than about 40° C.). However, certain problems have traditionally been associated with the use of such compositions where no solvent is present. For example, when one employs a slow-setting bitumen emulsion in the construction of load-bearing pavements the pavement is slow to cure and develop sufficient strength values adequate to support heavy traffic and to resist moisture damage. However, the use of a quick-setting bitumen emulsion often leads to premature breaking of the emulsion during handling and compaction (resulting in the formation of high air voids).
Various methods have been employed in attempts to accelerate the strength development of asphalt cold mixes made with bitumen emulsions. A number of these methods rely principally on shifting the pH of the emulsion or of the entire system in order to initiate or accelerate the break and coalescence of emulsion. U.S. Pat. No. 4,008,096 to Knapp (which is hereby incorporated by reference) is exemplary of this approach, teaching the injection of pH-shifting agents to initiate the coalescence of the emulsion.
However, these methods can suffer from a lack of control of the timing of the pH-shifting chemistry. In any construction process involving bitumen emulsions, the premature breaking or coalescence of the bitumen emulsions adversely affects performance in production, handling, transport, and other end-use application operations. Prematurely broken emulsion often fails to adequately coat aggregate and generally results in high-viscosity bitumen-aggregate mixtures which can be troublesome to discharge from trucks, may excessively stick to paving equipment, and which can be difficult to compact to required pavement densities. Thus, the failure to effectively control the timing of the pH-shifting chemistry commonly leads to premature rupture of the bitumen emulsion and a cascade of undesirable effects on mix processing and pavement applications.
Attempts have been made to control of the initiation of pH-shifting chemistries in bitumen emulsions. U.S. Pat. No. 5,256,195 teaches the use of anionic water-in-oil emulsion emulsions containing breaking agents in the aqueous phase, in combination with slow-setting cationic emulsions as the main mixing and coating emulsion. Upon mechanical action of compaction the water-in-oil emulsion ruptures to expose the alkaline aqueous phase to the cationic mixing emulsion, thereby shifting the pH of the system and initiating emulsion break. Those skilled in the art recognize that the use of mineral oils and other solvents (i.e., so-called “cutter stocks”) can lead to improvements in compactability. However, the use of cutter stocks can also result in decreases in the early compressive strength of compacted pavement. Dosages of cutter stocks as little as 0.1% by weight of the emulsion often decreases the compacted pavement compressive strength until such time as the cutter stock has evaporated into the atmosphere.
The formulation and production of fast-breaking bituminous emulsions by the use of rapid-setting (or spray-grade) or quick-setting emulsifiers is generally known, as disclosed in U.S. Pat. No. 4,338,136. However, approaches of this nature suffer from two drawbacks which make them unsuitable for construction of load-bearing asphalt pavements that exhibit complete aggregate coating, compact to required densities in the field, and develop strength rapidly. First, at ambient temperatures rapid-setting emulsions do not adequately coat dense-graded aggregates commonly used in construction of dense-graded, load-bearing pavements, since such rapid-setting emulsions tend to break immediately upon contact with mineral aggregate surfaces. The term “rapid-setting” is a defining characteristic of such emulsions (i.e., they immediately rupture and liberate water upon contact with mineral aggregate). Second, at ambient temperatures the immediate break of a rapid-setting emulsion produces a mix with high viscosity. The failure to adequately compact leads to low density pavements which fail under traffic due to deformation, disintegration, and/or, pot-hole formation (as water passes through the low density layer into the base where supporting pavement layers are degraded).
U.S. Pat. No. 5,743,950 discloses a process for preparing a bituminous composition by adding a hard bitumen binder having a penetration of less than 50 to a mixture of aggregates and a non-emulsified soft bitumen binder component. A hard binder component is defined as a binder component having a penetration of less than 200, as measured by ASTM D5 at 25° C. A soft binder component is defined as a binder component having a penetration of more than 700 as measured by ASTM D5 at 25° C. or a viscosity of less than 300 mPa·s. To obtain a bituminous composition with desired workability for load-bearing applications, an aggregate must be completely coated (i.e. mixed) with a soft binder prior to an addition of a hard binder. The bituminous composition must include both a hard bitumen binder and a soft bitumen binder. First, an aggregate is mixed with the soft bitumen to ensure a complete surface coating with the soft bitumen. Then, a hard bitumen is added to a mixture of aggregates and soft bitumen (i.e., aggregates surface coated with the soft bitumen) to impart stiffness and other desired properties for load-bearing applications. The hard bitumen may be added either in a form of powder or emulsion to mixture of a soft binder and an aggregate, namely an aggregated must be pretreated with a soft binder prior to a mixing with a hard binder. This process of producing bituminous composition requires a pretreatment of aggregate with soft binder prior to mixing with a hard binder; therefore, the equipment and process used to produce the bituminous composition must be designed to allow a pretreatment of aggregate.
Quick-setting emulsions are not suitable for the production of load-bearing asphalt pavement compositions at ambient temperatures for similar reasons. The use of large volumes of water beyond that present in the emulsion to promote coating of aggregate with quick-setting emulsions is not a feasible technique in the production of load-bearing pavements. First, load-bearing pavements are much thicker than the previously-mentioned slurry seal coatings and micro-surfacings. The thicker load-bearing pavements must be compacted to densify the mixture, as insufficient density can lead to rapid failure of load-bearing pavements due to deformation, disintegration, and pot-hole formation. Large water volumes prevent compaction in thick, load-bearing pavements to required densities because water is incompressible. Moreover, quick-setting emulsions develop high viscosity when stressed by high shear rate events such as compaction. As disclosed in U.S. Pat. Nos. 4,462,840 and 5,085,704, retarders are generally needed to slow down the break of quick-setting emulsion systems so that the materials might be handled and placed upon the intended construction surface prior to the development of such cohesive strength that they do not flow or cannot be spread.
Slow-setting emulsifiers are commonly employed in the production at ambient-temperature of emulsion-based road paving compositions for load bearing pavements. Slow-setting emulsifiers produce bituminous emulsions which require little or no water to achieve complete coating of the aggregate surface. Moreover, the slow-setting nature of the emulsion yields a road paving composition with a controlled coalescence rate, so that the road paving mixture does not increase in viscosity to a point that it is unsuitable for handling, hauling, or compaction. With highly dense, high-fines aggregate gradations, slow-setting emulsions do not break either prior to or during compaction, thereby rendering the mixture easy to compact at ambient temperatures. However, pavement compositions made at ambient-temperature with slow-setting bituminous emulsions are very slow to develop adhesion and cure to strengths sufficient to bear the stress of heavy traffic.
In contrast to cold mix paving compositions, hot mix paving compositions do not contain bitumen emulsions, but are instead produced by mixing non-emulsified bitumen with aggregate at elevated temperatures (usually in excess of 140° C.). The two most common hot mix facilities, drum mix plants and batch plants, heat aggregate in a rotating kiln to extremely high temperatures to drive off all water adsorbed to the aggregate, as well as all water absorbed within the surface pores of the aggregate. Quantitative removal of water is required (1) to ensure complete aggregates coatings, and (2) to ensure that the finished hot mixture of aggregate and bitumen shows no moisture sensitivity in the finished pavement layer once it is transported, laid down, and compacted.
Hot, dry aggregate produced in conventional hot mix operations is mixed with bitumen (which is previously liquefied by heating to temperatures far in excess of its melting point) to produce what is known in the industry as the “hot mix asphalt.” Hot mix asphalts generally must be produced, laid down, and compacted at temperatures in excess of about 160° C., as the compactability of the hot mix asphalt depends on the temperature. If the mix cools, the asphalt viscosity increases and the mixture cannot be compacted to the design density (known as percent air voids). When a hot asphalt-aggregate mixture cools to temperatures below about 85° C., the handling, placement, and compaction of the mixture become extremely difficult and design densities (air voids) cannot be realized.
Therefore, it is an object of the present invention to disclose a method of producing bituminous compositions.
It is another object of the present invention to provide bituminous compositions suitable for use in paving applications that are substantially free of volatile solvents.
It is yet another object of the present invention is to provide bituminous paving compositions that exhibit substantially complete aggregate coating and compactability to required densities in the field.
It is a further object of the present invention is to provide bituminous paving compositions that develop adhesive strength and load-bearing strength properties at rates comparable to traditional hot-mix paving compositions, and at rates faster than traditional cold-mix paving compositions.
It is still a further object of the present invention is to provide bituminous compositions that can be applied the surface being paved at temperatures lower than those required for conventional hot-mix bituminous compositions.
Other objects, features and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention.